1. Field of the Invention
The present invention generally relates to an imaging device, and more particularly to an imaging device which is applicable to reading optical systems of copy machines, facsimile machines and the like, an optical system of a reading scanner having a CCD sensor and a equimagnification sensor and optical systems of an optical printing head and a self-scanning type optical printing head.
2. Description of the related Art
In the recent years, it is required to miniaturize optical equipment, such as a copy machine and an optical printer head. To satisfy this requirement, a reading optical system and/or a writing optical system of the optical equipment have to be miniaturized. Thus, an equimagnification imaging optical system in which a distance between an object and an image can be strongly reduced is under investigation. The equimagnification imaging optical system is defined as an optical system which forms an image having the same size as an object.
A description will now be given of an example of the equimagnification imaging optical system having a conventional configuration. FIG. 1 illustrates the equimagnification imaging optical system having a conventional configuration. Referring to FIG. 1, a roof mirror lens array 103 is formed as the equimagnification imaging optical system. The roof mirror lens array 103 has a lens array 101 and a roof mirror array 102. The lens array 101 is formed of a plurality of lenses 104 which are arranged in line perpendicular to a a drawing plane of FIG. 1. The lenses 104 are optically equivalent to each other. The roof mirror array 102 is formed of a plurality of roof mirrors 106. The roof mirrors 106 are arranged in line so that each of the roof mirrors 106 faces one of the lenses 104. Each of the roof mirrors 106 has a ridge line 105. The ridge line 105 is perpendicular to a direction in which the roof mirrors 106 are arranged and an optical axis of each of the lenses 104. A stop member (not shown) is provided between the lens array 101 and the roof mirror array 102 so that imaging systems, each of which is formed of one of the lenses 104 and a corresponding one of the roof mirrors 106, are separated from each other.
A reading position P1 of an original 107 is set at a position which is not on the optical axis .phi. of each of the lenses 104 and corresponds to a finite slit height position. Light reflected from the reading position P1 of the original 107 passes through the each of the lenses 104 so that the light formed of parallel rays. The parallel rays travels to a corresponding one of the roof mirrors 106 and are reflected by the corresponding one of the roof mirrors 106 in the same direction. The light reflected by each of the roof mirrors 106 travels through a corresponding one of the lenses 104 again and is then focused on an imaging position P2 which is optically conjugate to the reading position P1. The position P2 is, for example, on a surface of a CCD sensor 108.
A prism lens array is disclosed in Japanese Patent Publication No.61-2929. Into this inprism lens array, a lens array and a roof mirror lens array are integrated. In the same manner as the roof mirror lens described above, a reading position is set at a position corresponding to a finite slit height position. The light reflected at the reading position travels through each of lenses and is then reflected by each of roof prisms twice. The light reflected by the each of the roof prisms travels through a corresponding one of the lenses again and is focused on an imaging position which is optically conjugate to the reading position.
A roof mirror lens array which is the equimagnification imaging optical system is disclosed in Japanese Laid-Open Patent Application No.57-37326. Into this roof mirror lens array, a lens array, a roof mirror array and a stop member are integrated. The lens array has lenses which are optically equivalent to each other. The lenses are arranged in line. The roof mirror array has roof mirrors. Each of the roof mirrors faces one of the lenses and has a ridge line. The ridge line is perpendicular to a direction in which the lenses are arranged and to an optical axis of each of the lenses. The stop member is provided between the lens array and the roof mirror array to separate imaging optical systems each of which is formed of a corresponding one of the lenses and a corresponding one of the roof mirrors. The roof mirror lens array may be used to read images and for exposure of a photosensitive member.
In each of the imaging devices as described above, a single imaging system is formed of a lens of the lens array and a roof mirror of the roof mirror array. An aperture of the stop member is provided between corresponding lens and roof mirror to optically separate the imaging system from adjacent imaging systems. In this type of the imaging device, the light travels and returns through the lens. Thus, is not possible to locate the reading position and the imaging position at the same position. The light rays travels along the optical axis are separated to an object (the original) side rays and imaging point side rays. Thus, the reading position and the imaging position have to be set based on a finite slit height position. That is, the reading position P1 is set at a finite height position in a direction parallel to the ridge line 105 of each of the roof mirrors 106. The imaging position P2 is set at the finite height position in the reverse direction.
Since the amount of separation of the light rays is limited, separation mirrors 109(1) and 109(2) are used to set the reading position P1 and the imaging position P2 as shown in FIG. 1. The light traveling from the reading position P1 is reflected by the separation mirror 109(1) and travels to a corresponding one of the lenses 104. The light passing through each of the lenses 104 is reflected by the separation mirror 109(2) and focused on the imaging position. Each of the separation mirrors 109(1) and 109(2) is a rectangular plane mirror which expands in a direction perpendicular to the drawing plane of FIG. 1. Each of the separation mirrors 109(1) and 109(2) are arranged so as to be inclined by 45.degree. with respect to a plane including optical axes .phi. of the lenses 104 of the lens array 101.
In the conventional imaging device having a roof mirror lens or a roof mirror lens array, the light passes through the same lens 104 twice, and the reading position P1 (a reading plane) and the imaging position P2 (an imaging plane) are located in the opposite sides with respect to the optical axis .phi. of the lens 104. The separation mirrors 109(1) and 109(2) are provided in optical paths between the reading position P1 and the lens 104 and between the lens 104 and the imaging position P2.
The surfaces of each roof mirror and the separation mirrors 109(1) and 109(2) are provided with reflecting films which are formed of high reflecting material, such as aluminum (Al), by a vacuum evaporation process. The reflectivity of each of the reflecting films is about 90%. In the imaging device having the above structure as shown in FIG. 1, there are two reflecting surfaces of each of the roof mirrors 106 and two reflecting surfaces of the respective separation mirrors 109(1) and 109(2). Thus, the total reflectivity of is about 66%. The loss of the amount of light in the imaging device is large.
In addition, in the conventional case, the light pass through the same lens 104 twice, so that the reading position P1 and the imaging position P2 have to be adjacent and to be symmetrical to each other with respect to the optical axis .phi.. Thus, stray light, such as reflected light from the surface of the lens 104 and from surfaces other than the reflecting surface of the roof mirror 106, may be incident on the imaging position P2 at a high possibility. Such stray light affects characteristics of optical images. In general, the contrast and the resolution of the optical images deteriorate.
Further, FIG. 2 illustrates an essential part of another example of the conventional imaging device. Referring to FIG. 2, the imaging device has a lens array 121 and a roof mirror array 122. Each of roof mirror of the roof mirror array 122 has a ridge line portion 122a between arranged optical axes. A roof mirror lens array is formed of the lens array 121 and the roof mirror array 122.
Each of the roof mirrors of the roof mirror array 122 has two reflecting surfaces which are connected to each other at an angle of 90.degree. so that the ride line portion 122a is formed. However, light L' which is obliquely incident on each lens of the lens array 121 is reflected by a corresponding one of the roof mirrors twice and then ejected from an adjacent lens. That is, the light L' obliquely incident on an optical system is ejected from an adjacent optical system in the imaging device.